/*
定义一个点类Point,并定义成员函数double Distance(const & Point) 求两点的距离。
输入两个点的坐标，
创建两个点， 然后调用Point类的Distance方法输出两个点的距离。*/
#include<iostream>
#include<math.h>
using namespace std;

class Point
{
public:
	int a, b;
	double Distance(Point B);
	Point(int x,int y):a(x),b(y){}

};

double Point::Distance(Point B)
{
	double x, y;
	double ret;
	x = a - B.a;
	y = b - B.b;
	ret = sqrt(x * x + y * y);
	return ret;
}


int main() {
	double a, b, c, d;
	cin >> a >> b >> c >> d;
	Point A(a, b), B(c, d);
	cout << A.Distance(B) << endl;
	return 0;
}